Thin Fabrics and Films to Hydrogen and Helium. 273 

 II. Preliminary Experiments, 



In order to test the apparatus and to acquire a working 

 familiarity with the instruments, a preliminary study of the 

 diffusion of hydrogen through the fabrics used bv Elworthy 

 and Murray was made. The apparatus used and the method 

 of assembling it was the same as described in their paper. 

 The fabrics used b\ r them were inserted as a separating 

 diaphragm in an air-tight drum-like vessel. Two gases were 

 brought into this drum, one on either side of the fabric, and 

 their transfusion was determined by tests on the gases by 

 means of the instruments mentioned above. For a full 

 description of the Shakespear apparatus the reader is referred 

 to the paper by Elworthy and Murray. It will suffice here 

 to say that this apparatus was made by the Cambridge 

 Scientific Instrument Co., and that its principle is based on 

 the variation in resistance of a heated platinum coil, consti- 

 tuting one branch of a Wheatstone Bridge circuit, when the 

 gas mixture surrounding the cell has its thermal conductivity 

 varied by changes in its component parts. The two methods 

 adopted were (1) to pass a continuous stream of pure air and 

 one of pure hydrogen on opposite sides of the fabric as a 

 dividing diaphragm, and (2) to enclose a known quantity of 

 pure air on one side and to pass a continuous stream of pure 

 Irydrogen past the other side of the fabric. 



In the present experiments both methods were followed, 

 but gas tests were made with the katharometer only. It 

 was found that 20° C. was a more suitable temperature for 

 working at than 15 o, C. as previously used by Elworthy 

 and Murray. The measurements obtained were made by 

 keeping the permeameter and connexions in a thermostat 

 at 20 o, C, the variation in temperature being not more 

 than 0°-2 C. 



III. Calibration, 



The katharometer used to detect small percentages of 

 hydrogen or of helium in air had already been calibrated for 

 both gases ; but this calibration was checked by noting the 

 galvanometer deflexions for a given sample of gas, deducing 

 the percentage of helium or hydrogen present from the 

 calibration curve and then checking the result by actually 

 weighing a known volume of the sample studied. It was 

 found that the values obtaineJ by the latter method fitted in 

 very closely with the calibration curve of Elworthy and 

 Murray. It may be stated here that in their work it had 



